http2: don't log about timeouts reading client preface on new connections
http2: don't log about timeouts reading client preface on new connections

Updates golang/go#18776

Change-Id: I7568f779e2b86c72c54c8744c08cc02988dde55b
Reviewed-on: https://go-review.googlesource.com/79498
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Tom Bergan <3b77d7da34779f829901a664ac7c87d255b041af@google.com>

golang/net